[PATCH v3 4/9] completion: Replace spin_unlock_wait() with lock/unlock pair

From: Paul E. McKenney
Date: Fri Jul 07 2017 - 15:29:11 EST

There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair. This commit therefore replaces the spin_unlock_wait() call in
completion_done() with spin_lock() followed immediately by spin_unlock().
This should be safe from a performance perspective because the lock
will be held only the wakeup happens really quickly.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
kernel/sched/completion.c | 9 ++-------
1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index 53f9558fa925..3e66712e1964 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -307,14 +307,9 @@ bool completion_done(struct completion *x)
* If ->done, we need to wait for complete() to release ->wait.lock
* otherwise we can end up freeing the completion before complete()
* is done referencing it.
- *
- * The RMB pairs with complete()'s RELEASE of ->wait.lock and orders
- * the loads of ->done and ->wait.lock such that we cannot observe
- * the lock before complete() acquires it while observing the ->done
- * after it's acquired the lock.
- smp_rmb();
- spin_unlock_wait(&x->wait.lock);
+ spin_lock_irq(&x->wait.lock);
+ spin_unlock_irq(&x->wait.lock);
return true;